The present invention relates generally to II-VI compound semiconductor epitaxial layers such as ZnSe and Zn (S.sub.x Se.sub.1-x) having a low incidence of lattice defects, and also relates to a method for producing such layers and to devices utilizing such layers, for example, light emitting diodes and semiconductor injection lasers.
There is a continuous push toward higher data densities on optical discs. In particular, for digital storage of video data, a substantial increase over the presently attainable densities must be achieved. The information density on optical discs is largely limited by the bit size, which is in turn determined by the wavelength of the laser used to read and write the data. Current optical disc systems use infrared emitting III-V compound semiconductor lasers operating in the wavelength range of 800 to 850 nm. Decreasing the wavelength by a factor of 2, by using blue lasers operating at half the wavelength of presently used III-V lasers, could halve the laser spot diameter and therefore allow a four-fold increase in the information density. ZnSe is the most promising material so far for producing practical blue emitting semiconductor injection lasers. Such lasers emit at a wavelength near 460 nm.
As shown in FIG. 1a, a basic laser structure 10, the so-called double heterojunction semiconductor injection laser, contains an active layer 17 sandwiched between two cladding layers 15 and 19 of a wider bandgap material. Laser action takes place in the active region where electrons and holes recombine to generate coherent light. Electrons flow from the n-type cladding layer into the conduction band of the active layer. Holes flow from the p-type cladding layer into the valence band of the active layer. This process is illustrated in the energy band diagram of FIG. 1b. As is known, electrons and holes can be injected electrically where the cladding layers are doped n-type and p-type, or can be injected by optical or electron beam pumping, in which cases it is not necessary for the layers to be doped.
The cladding layers must have a bandgap higher than that of the active layer to assure effective injection of electrons and holes, and to minimize the threshold for laser action. The cladding layers also have a lower index of refraction. Therefore, the light generated is confined to the active layer, which guides the light in an intended direction, and also helps to minimize the laser threshold.
The bandgap of the semiconductor in the active layer determines the wavelength of the light generated. The II-VI semiconductors ZnSe and ZnS have relatively wide bandgap energies of 2.7 and 3.7 ev, respectively, and are therefore suitable materials for building short wavelength blue lasers. However, lattice defects in these and other laser materials, particularly stacking faults and threading dislocations, are known to adversely affect the operation of these semiconductor lasers, and it has been found to be difficult in practice to obtain epitaxial layers of the II-VI compounds with an acceptably low incidence of lattice defects by conventional deposition techniques such as molecular beam epitaxy (herein "MBE").